1. Field of the Invention
The invention relates to a device for feeding a pressing apparatus, to a pressing apparatus and to a pressing method.
More particularly, the field of the present invention relates to the dry pressing of ceramic material and even more particularly to the dry pressing of tiles.
In these pressing operations, the mold of the press must be filled with a ceramic material, in the form of granular or powderlike material, preferably of the so-called xe2x80x9catomizedxe2x80x9d type; then the upper surface of the mold, particularly for the pressing of tiles, is skimmed so as to obtain a flat surface of the material to be pressed; then the upper plunger is lowered and the material is dry-pressed. The formed material is then extracted from the mold and sent to the subsequent firing process.
2. Description of the Prior Art
According to the known art, the mold is loaded by means of a grille which can slide above and outside the mold. When the grille is outside the mold, it is loaded with the material to be pressed and is then made to slide above the mold, so that the material to be pressed fills the mold by gravity. Then, by means of the return motion of the grille, the material is skimmed on the surface of the mold.
This system generally has the problem of producing differences in the density of the material inside the mold, since the distribution of the powder in the grille depends on the movement of the grille above the mold and therefore the part of the mold that is skimmed last has a higher density than the rest. This uneven density gives rise to deformations during the firing of the part and therefore increases rejects.
This known method furthermore has problems when it is necessary to produce decorations having an aesthetic value on the tiles. The grille mixes the powder in its back-and-forth movement, accordingly limiting the aesthetic effects that can be obtained. Furthermore, in order to produce these decorations it is known to first load an atomized foundation of material and then, after a second pass, load a decorated surface layer. The decorated surface layer can be applied either by means of a second grille or by means of a retractable ribbon onto which the material has been loaded, in the required color composition, with a screen-printing screen. However, the use of a second grille still leaves the impressions of said grille in the final aesthetic result, producing an unwanted effect, whilst the retractable ribbon still entails a free fall of the material from the ribbon onto the mold, thus mixing the material and generating difficulties in obtaining decorations having the intended level of detail. The system is furthermore rather slow and this entails a reduction in the productivity of the plant.
Furthermore, during the return stroke for skimming the mold, a layer of powder of mixed colors is deposited over the decorated layer due to the inertia effects of the system; accordingly the finished tiles must be treated, after firing, in order to remove the surface layer, which is approximately 1 mm thick. This entails a long and expensive treatment which significantly affects the price of the end product.
Furthermore, the fact that the decorated layer is relatively thin with respect to the atomized foundation entails the fact that after tile laying the polishing of the floor by means of machines which use abrasives must be limited of necessity, in order to avoid fully removing the decorated surface layer.
It should also be noted that this system entails a high waste of ceramic powder, which accumulates below the carriage that moves the grille because the system requires an excess of powder by definition.
During the second loading operation using the colored material, said colored material is furthermore inevitably deposited only in the regions to be colored. This entails an accumulation of material and a higher density in the colored regions with respect to those that are not colored. These differences in density entail, during firing, deformations of the tiles, which are either rejected or must be subjected to grinding, entailing a significant cost increase.
Owing to the above cited problems it is impossible to obtain the decoration on the raw unpolished material, whereas the market, particularly the foreign market, requires to provide the decoration on the unpolished surface in order to improve antislip performance.
According to WO 98/23424, GB 2 310 161 and Patent Abstract of Japan vol. 017, no. 552 (M-1491). Oct. 5, 1993, JP 05 154827 it is known to use a roller provided with grooves to fill the grooves with a powder, so that a rotation of the roller causes the pouring of the powder on the article to be decorated. However this arrangement allows only a decoration of the surface of the article. It is impossible to have the full body of the article to contain the decoration. Again, the fact that the decorated layer is relatively thin with respect to the atomized foundation entails the fact that after tile laying the polishing of the floor by means of machines which use abrasives must be limited of necessity, in order to avoid fully removing the decorated surface layer. Also a single decorating material can be used. Also the relatively high thickness of the decorated parts cause density variations inside the tile, so that, during firing the tile is suitable to deform irregularly.
The aim of the present invention is therefore to overcome the above cited drawbacks with a device for feeding at least two different granular or powderlike materials to a pressing apparatus for the dry pressing of said materials, comprising at least two containers for said materials and at least two rollers arranged each at an opening of one of said containers; each of said rollers having a cavity which is suitable to be filled by one of said materials, by means of said opening, and being suitable to be freed of said material following a rotation of said roller so as to pour said material into a mold, so as to be able to generate colorings and/or shades and/or aesthetic effects as required.
The invention furthermore relates to a pressing apparatus which comprises a device as described above. For a description of a pressing apparatus which can be used with a device as described above, reference is made to European patent 0 422 041 in the name of the same Applicant.
According to another aspect, the invention furthermore relates to a method for the dry pressing of at least two distinct granular or powderlike materials, comprising the loading of a mold by virtue of the rotation of at least two rollers each one of which contains one of said materials in a cavity formed in itself; a first of said rollers loading certain parts of said mold; a second of said rollers loading other parts of said mold, that have been left free by said first roller; so as to generate colorings and/or shades as required.
Preferably, during the process the following operations are performed simultaneously:
a) a rotation of the roller and
b) a relative movement of said rollers with respect to said mold, at right angles to the axis of said rollers;
so that the material is distributed in the mold as required.